1. Field of the Invention
This invention pertains to an electron beam apparatus for measuring a voltage at an irradiation point by irradiating a primary electron beam through an objective lens on a sample, such as an internal wire of a semiconductor device like an LSI, and by analyzing a secondary electron e2 emitted from the irradiation point. More specifically, it pertains to an improvement in an in-lens type secondary electron energy analyzer provided inside of an objective lens by reducing the aberration of an objective lens with an aim of narrowing its objective lens magnetic field distribution.
2. Description of the Related Arts
For analyzing an internal operation, for example, of an LSI, an electron beam is used for measuring the voltage waveform of an internal wire. For measuring a voltage waveform, a secondary electron energy analyzer measures the energy of a secondary electron e2 emitted from the internal wire on which a primary electron beam is irradiated. As an LSI is ever more densely integrated, an electron beam used for this purpose needs to be reduced in size. Generally, the shorter a focal length is, the smaller a lens aberration is, which enables a smaller beam size to be formed. Hence, being developed is a new in-lens type secondary electron energy analyzer capable of reducing an aberration by shortening the focal length of an objective lens.
A number of variations in in-lens type secondary electron energy analyzers have been publicly known, which can be broadly classified into two types.
A conventional in-lens type secondary electron energy analyzer of a first kind is a system of having a plurality of planar grids (mesh electrodes) analyze the energy of a secondary electron e2, as disclosed e.g. in Japanese patent publications Tokukaisho 58-192255, 58-197644, 63-78444, 64-10562 and Tokukaihei 01-200546.
A conventional in-lens type secondary electron energy analyzer of a second kind is a system of having a spherical grid (mesh electrode) analyze the energy of a secondary electron e2 having been accelerated by a planar grid (mesh electrode) or a pinhole grid (aperture electrode), as disclosed, for example, in Japanese patent publications Tokukaisho 59-90349, 60-146439, 61-288357.
FIG. 1 shows, as an example of the first kind, an apparatus disclosed in Japanese patent publications Tokukaihei 01-200546.
FIG. 3 shows, as an example of the second kind, an apparatus disclosed in Japanese patent publications Tokukaisho 61-288357.
In either system, an improvement in the accuracy in analyzing the total energy of a secondary electron e2 emitted radially from an electron beam irradiation point is required.
Accordingly, a conventional in-lens type secondary electron energy analyzer of the first kind has its objective lens 1 collimate (make parallel) a secondary electron e2, thereby injecting it into the planar surface of a retarding grid (mesh electrode) 4 as perpendicularly as possible. Likewise, a conventional in-lens type secondary electron energy analyzer of the second kind has its objective lens 11 focus a secondary electron e2, thereby injecting it into the spherical surface of a retarding grid (mesh electrode) 14 as perpendicularly as possible.
FIG. 1 is a squint view illustrating a cutaway of a conventional in-lens type secondary electron energy analyzer, having a plurality of planar mesh electrodes for a magnetic field collimation of a secondary electron e2.
A conventional in-lens type secondary electron energy analyzer of the first kind comprises three planar mesh electrodes, i.e. an extraction grid 2, a collimation grid 3 and a retarding grid 4, inside of an objective lens 1.
FIG. 2A is a graph illustrating a magnetic field distribution caused by an objective lens 1 of the conventional in-lens type secondary electron energy analyzer shown in FIG. 1.
Here, the collimation grid 3 is placed at a position having about a half of the peak value in the right hand side on the transverse axis of a magnetic field distribution curve of the objective lens 1, i.e. at a position spaced by a distance ZC from the surface of a sample 5, and is applied with a voltage (e.g. one hundred volts) sufficiently lower than one applied at the extraction grid 2. With this configuration, when a primary electron beam e1 is irradiated on the surface of the sample 5 after being focused by the objective lens 1, the irradiation point emits a secondary electron e2. The collimation grid 3 decelerates the secondary electron e2 thus emitted from the surface of the sample 5 and accelerated to have a kinetic energy of several hundred to about a thousand electron volts by the extraction grid 2. As a result, the right hand side of the transverse axis of the magnetic field distribution curve of the objective lens 1 has a stronger refraction for secondary electrons.
FIG. 2B is a graph illustrating the trajectory of a secondary electron e2 passing through the conventional in-lens type secondary electron energy analyzer shown in FIG. 1.
For the above reason, theoretically, the secondary electron e2 emitted from the surface of the sample 5 falls perpendicularly on the retarding grid 4, thus undergoing a accurate energy retarding independent of its emission angle from the surface of the sample 5. A secondary electron detector 6 detects the secondary electron e2 that has passed through the retarding grid 4, after being separated into those capable and those incapable of passing through the retarding grid 4.
Here is a secondary electron focuses at one point (in FIG. 2B shown as the point F). This comes from the influence of an objective lens. A secondary electron is collimated after focusing at this point.
FIG. 3 is a cross-sectional view illustrating a conventional in-lens type secondary electron energy analyzer, having a plurality of spherical mesh electrodes whose center resides at the focus of an objective lens 11 for a secondary electron e2, i.e. at the point F.
A conventional in-lens type secondary electron energy analyzer of the second kind comprises an extraction electrode 12 inside of an objective lens 11, a mesh electrode 13 spherically provided outside of the objective lens 11, where these two electrodes, i.e. the extraction electrode 12 and the mesh electrode 13 are applied with the same voltage in a range between one and two kilovolts, and a retarding mesh electrode 14 provided spherically further outside of the mesh electrode 13.
With this configuration, when a primary electron beam e1 is irradiated on the surface of the sample 15 after being focused by the objective lens 11, the irradiation point emits a secondary electron e2. The objective lens 11 refocuses the secondary electron e2 thus emitted from the surface of the sample 15 at its focal point F before it is diverged, where the focal point F is also the center of these two spherical electrodes, i.e. the mesh electrode 13 and the retarding mesh electrode 14. Therefore, theoretically, the secondary electron e2 emitted from the surface of the sample 15 falls perpendicularly on the spherical surface of the retarding mesh electrode 14, thus undergoing an accurate energy retarding independent of its emission angle from the surface of the sample 15.
However, a conventional in-lens type secondary electron energy analyzer of the second kind has a disadvantage of these two spherical mesh electrodes, i.e. the mesh electrode 13 and the retarding mesh electrode 14, necessarily having larger external dimensions, because it causes the secondary electron e2 diverged from the surface of the sample 15 to converge once at the focal point F before radially diverging to fall perpendicularly on the spherical surface of the mesh electrode 13 and the retarding mesh electrode 14.
Hence, a provision of these two spherical grids, i.e. the mesh electrode 13 and the retarding mesh electrode 14, outside of the objective lens 11 requires an extra space. On the other hand, a provision of these two spherical mesh electrodes, i.e. the mesh electrode 13 and the retarding mesh electrode 14, inside of the objective lens 11 requires the objective lens 11 to have a larger magnetic circuit. In either case, a conventional in-lens type secondary electron energy analyzer of the second kind presents a difficulty in reducing the overall dimensions of the whole apparatus.
The description of the related arts continues by returning to FIG. 1.
On the other hand, a conventional in-lens type secondary electron energy analyzer of the first kind has an advantage of the three planar mesh electrodes, i.e. the extraction grid 2, the collimation grid 3 and the retarding grid 4, being small, because it causes the secondary electron e2 divergently emitted from the surface of the sample 5 and collimated by the collimation grid 3 to fall perpendicularly on the planar surface of the retarding grid 4.
Hence, a provision of these three planar mesh electrodes, i.e. the extraction grid 2, the collimation grid 3 and the retarding grid 4, requires a small space, because a collimated secondary electron e2 does not diverge randomly in all directions. Thus, it is easy to provide the three planar mesh electrodes, i.e. the extraction grid 2, the collimation grid 3 and the retarding grid 4, inside of the objective lens 1, and the secondary electron detector 6 need only be provided outside of the objective lens 1.
As a result, a conventional in-lens type secondary electron energy analyzer of the first kind has an advantage of having overall dimensions smaller than one of the second kind. Additionally, it has a further advantage that the fabrication of a planer mesh electrode is much simpler and more precise than that of a spherical mesh electrode.
Yet, a conventional in-lens type secondary electron energy analyzer of the first kind has a following disadvantage in shortening the focal length for reducing the dimensions of a beam aperture by further reducing the aberration of the objective lens 1.
First, since an LSI package partly uses a magnetic material, a generic electron beam apparatus for a voltage measurement has a limit on the intensity of a leakage magnetic field on the surface of the sample 5, because of the increase of an astigmatism caused by the magnetic material.
A lens focal length is, simply put, proportionate to the extent of the magnetic field distribution and inversely proportionate to the square of the maximum value of a magnetic field distribution. Because of the magnetic nature of the LSI package, it becomes necessary to have the intensity of the magnetic distribution of the objective lens 1 to be limited. To shorten the focal length, this reduces the extent of magnetic distribution contributing the collimation of a secondary electron e2. Hence, a secondary electron e2 needs to have its speed (i.e. kinetic energy) lowered sufficiently to be able to be collimated.
Meanwhile, in conducting the voltage measurement of an internal wire of an LSI with an insulation cover film, because of a need for preventing the insulation cover film from being charged up, the voltage applied at the extraction grid 2 for accelerating a secondary electron e2 must be lowered by several hundred volts.
As a result, an optimal voltage for the collimation grid 3, which is almost proportionate to a voltage Ve applied to the extraction grid 2, draws dramatically down to a range between fifteen volts and twenty volts.
For this reason, when the voltage at the collimation grid 3 draws down to a range between fifteen volts and twenty volts, a variation of the sample voltage having an order of magnitude at several tens of volts generated in the case of an insulation cover film, causes the trajectory of a secondary electron e2 inside of the secondary electron analyzer to be changed greatly, which produces a large diversion from a collimation condition enough to disable a precise energy retarding.
FIGS. 4A through 4F show a concrete example of such problems in a conventional in-lens type secondary electron energy analyzer of the first kind.
FIG. 4A is a graph illustrating the relative positions of the surface of the sample 5, the extraction grid 2, the collimation gird 3 and the retarding grid 4 of the conventional in-lens type secondary electron energy analyzer of the first type shown in FIG. 1.
FIG. 4B is a graph illustrating the trajectory of a secondary electron e2 passing through the conventional in-lens type secondary electron energy analyzer of the first type shown in FIG. 1, when a sample voltage Vsp is minus twenty 20 volts.
FIG. 4C is a graph illustrating the trajectory of a secondary electron e2 passing through the conventional in-lens type secondary electron energy analyzer of the first type shown in FIG. 1, when a sample voltage Vsp is zero volts.
FIG. 4D is a graph illustrating the trajectory of a secondary electron e2 passing through the conventional in-lens type secondary electron energy analyzer of the first type shown in FIG. 1, when a sample voltage Vsp is twenty volts.
FIG. 4E is a graph illustrating an axial magnetic flux density distribution on an optical axis Z.
FIG. 4F is a graph illustrating an electric potential distribution on an optical axis Z.
It is assumed here that voltages Ve applied at the extraction grid 4 and Vc applied at the collimation grid 3 are three hundred seventy volts and twenty volts, respectively, and that the voltage Vr applied at the retarding grid 4 is set equal to the sample voltage Vsp applied to the sample 5. Because the FWHM (full width of half maximum) of the magnetic field distribution is set to about a half of the convention, the focal length is shortened, thereby reducing the aberration. Yet, the magnetic field is only effective as a lens in focusing a secondary electron e2 to a position (the focal point F in FIGS. 4B, 4C and 4D) for focusing a secondary electron e2 midway between the extraction grid 2 and the collimation grid 3.
Accordingly, although FIG. 4C shows a case in which it is possible to collimate a secondary electron e2 when the sample voltage Vsp is zero volts, it is impossible to collimate a secondary electron e2, i.e. to inject a secondary electron e2 perpendicularly on the retarding grid 4, when the sample voltage Vsp is minus twenty volts as in a case shown in FIG. 4B or when the sample voltage Vsp is twenty volts as in a case shown in FIG. 4D.
Especially, because FIG. 4F shows a case in which the electric potential at the position of the collimation grid 3 on the optical axis Z (the potential at a point C) is not more than twenty volts when the sample voltage Vsp is twenty volts as in a case shown in FIG. 4D, a secondary electron e2 emitted from the sample 5 applied with the sample voltage Vsp at twenty volts may not be able to pass through point C in the worst case.
As well, it is apparent that a collimation is effective only between the collimation grid 3 and the retarding grid 4 when the sample voltage Vsp is zero volts, and that this is due to the effect of a nonuniform electrical field distribution actually in place despite the theoretical presence of a uniform electrical field, instead of due to the effect of a magnetic field.
FIG. 5A is a graph illustrating an integrated energy spectrum of secondary electron (so called S-curve) obtained by the conventional in-lens type secondary electron energy analyzer shown in FIG. 1, when sample voltages Vsp are zero volts and three volts.
FIG. 5B is a graph illustrating a S-curve obtained by the conventional in-lens type secondary electron energy analyzer shown in FIG. 1, when sample voltages Vsp are ten volts and thirteen volts.
These S-curves are obtained by having the secondary electron detector 6 detect a secondary electron e2 passing through the retarding grid 4 when the voltage applied to the retarding grid 4 is changed from minus twenty volts to twenty volts. It is necessary here for a secondary electron e2 to fall perpendicularly on the retarding grid 4, which decelerates, halts or even returns a secondary electron e2. A secondary electron e2 may not pass through the retarding grid 4 even if it does not halt to a complete stop, when a secondary electron e2 does not fall perpendicularly on the retarding grid 4.
This is because the retarding grid 4 does not change the speed of a secondary electron e2 vectored parallelly to the planar surface of the retarding grid 4, even though the retarding grid 4 reduces the speed of a secondary electron e2 vectored perpendicularly to the planar surface of the retarding grid 4.
This means that a secondary electron e2 having a sufficient speed (kinetic energy) to pass through the retarding grid 4 may not pass through the retarding grid 4 when its trajectory is not perpendicular to the planar surface of the retarding grid 4. This should be interpreted as a failure in correctly analyzing the kinetic energy of a secondary electron e2.
That is, a conventional in-lens type secondary electron energy analyzer of the first kind can correctly analyze the kinetic energy of a secondary electron e2 only when a secondary electron e2 falls perpendicularly on the retarding grid 4 after having been collimated properly, because the S-curve has a shape of integrating the energy distribution of a secondary electron e2.
FIG. 5A illustrates a three volt shift, which indicates a correct energy retarding. But FIG. 5B illustrates an energy retarding curve far from a proper one that should be obtained by integrating the energy distribution of a secondary electron e2, which disables a shift between two curves, i.e. a voltage shift, to be measured.
That is, the conventional in-lens type secondary electron energy analyzer of the first kind (shown in FIG. 1) having a plurality of planar grids (mesh electrodes) for a magnetic field collimation of a secondary electron e2, has its collimation grid 3 slow down the speed of a secondary electron e2 in a vicinity of the magnetic distribution generated by a magnetic field lens, and enables a secondary electron e2 to be collimated by adjusting the effect of a magnetic field distribution for a secondary electron e2.
However, a narrow magnetic field distribution causes a secondary electron e2 to once converge at a point devoid of a substantial magnetic distribution. Theoretically, a mere posterior adjustment of the speed of a secondary electron e2 is not sufficient to collimate it.
It is conceivable to newly add a magnetic field distribution for collimating a secondary electron e2 shown as a dashed line in FIG. 4E. Yet, because the newly added magnetic distribution affects a primary electron beam e1 greatly, this brings about an effectual spread of a lens magnetic distribution and fails to reduce an aberration. It is also conceivable, as another method, to shift the position of the collimation grid 3 closer to the surface of the sample 5, thus slowing down the speed of a secondary electron e2 in a range having a magnetic field distribution, thereby improving the collimation performance. But a lower voltage at the main part of a magnetic field distribution increases a lens aberration.